Did two things: 1. Got my caffeine intake to zero over a month. Basically, no coffee or tea (don't drink sodas or anything else) for four weeks. 2. Started to have a strong espresso with two L-Theanine pills very morning before workout.

Noticed a much improved ability to concentrate on mundane tasks, and much less procrastination. I think, effect lasted for some 6-8 weeks, after that got off by drinking coffee during the day as well, so not sure what actually stopped the effect - caffeine or l-theanine tolerance.

I've been on and off L-theanine in different forms but regulated regimens for around 8 years. Of course my experience is just anecdotical and applies only to my metabolism and psychology but your case sounds like what happens when I get tolerance to L-theanine. The capsules do that for me pretty fast, but if I rotate between L-theanine rich Korean Sencha, Oolong and Gyokuro/matcha the effect never goes away.

I only need to not get too high on caffeine and to not repeat the same tea so much, if I do when I realise and switch to the other one the effect comes back.

Getting Oolong's and senchas with good theanine content is hard, specially finding one that works for you. Glutamate rich smell is a good indicator as well as the color or whiteish spots. Gyokuro or matcha is a safer source but if you only use that you get the tolerance thing too.

Yellow bag Lipton also works a little bit for me but not as effectively.

Is there a reason to believe the L-Theanine was a factor there? What you describe sounds exactly like the normal effects of caffeine.

L-Theanine makes caffeine's effects last much longer.

As gp said, 1 espresso, 6 hours.

For me it also cuts down the side effects, all around a very useful substance.

It gives you a calm focus. Sometimes (rarely) I realize I'm cranky for no reason after I respond to something with more emotion than it deserved, which isn't good on relationships. L-theanine works great for me to take the edge off.

I take 400mg before interviews when I'm job hunting. Unlike benzodiazepines, it doesn't seem to affect my problem solving ability at all. The effect is pretty mild compared to stronger anti-anxiety medication, but for me it seems to take my anxiety down a couple notches.

During a regular onsite interview, without l-theanine I might have some heart racing, and I might not be able to focus well enough to even understand what an interviewer is asking me. With l-theanine, the anxiety is still there but I can at least understand the problems I'm given.

IIRC it doesn't have any effect by itself but makes the effect of the caffeine less "jittery"